In the conventional processing of the copper clad laminate into a printed wiring board, etching by solution has been mainly employed. Therefore, sufficient adhesion performance has been required not only between a copper foil and an insulating resin substrate as a copper-clad laminate but also between a circuit and the insulating resin substrate after processing into a printed wiring board.
In order to satisfy such requirements, many efforts to apply a surface treatment to a bonding surface of the copper foil for manufacturing a printed wiring board have been made as a measure to enhance adhesion performance to the insulating resin substrate. In addition, a chromium component has been widely used as a rust-proofing element or an element to change surface properties in the conventional copper foils for manufacturing a printed wiring board utilizing the manners, chromium plating, chromate treatment or the like. In particular, chromate treatment has been applied to the recent copper foils shipped to the market.
Examples in which a chromium component is used as a surface-treatment component will be described. For example, Patent Document 1 discloses copper foils for a printed wiring board superior in adhesion performance (bond strength between base material and copper foil), moisture resistance, chemical resistance and heat resistance. One of the copper foils has a vapor deposited metal chrome layer on one surface or both surfaces of copper foil, for example, vapor-deposited chromium layer formed by a sputtering method. And another one of copper foils is supported the one surface on a carrier via a release layer and the opposite surface of the copper foil has a vapor-deposited metal chrome layer, for example, vapor-deposited chromium layer formed by a sputtering method.
Patent Document 2 discloses a copper foil in which the performance of a silane coupling agent for increasing the bond strength of a copper foil used for manufacturing a printed wiring board to a substrate is enhanced in maximum. The surface-treated copper foil for a printed wiring board is manufactured by forming a zinc or zinc alloy layer on a surface of a copper foil, and forming an electrolytic chromate layer on the surface of the zinc or zinc alloy layer as a rust-proofing treatment, and then a silane coupling agent-adsorbed layer is formed on the electrolytic chromate layer without drying the electrolytic chromate layer, and finished by drying.
When the chromium component used as a component for surface treatment described above exists as a chromium compound, oxidation number is trivalent or hexavalent. In addition, as for biological toxicity, hexavalent chromium is much higher and mobility in the soil is faster in hexavalent chromium and is heavier in environment load.
With regard to the wastes containing a hazardous substance harmful to human body such as chromium, transboundary movements have been occurred world-wide since 1970s. As a result, problems such as an environmental pollution are generated by a hazardous waste shipped from industrialized countries and left in developing countries or the like. Then, “Basel Convention on the Control of Transboundary Movements of Hazardous Wastes and their Disposal” was created in 1980s to establish international framework and procedures on the regulations of transboundary movements of certain types of wastes. The convention came into force in 1993 in Japan.
In recent years, the ELV directive of EU (European Union) has prohibited the use of specified substances with a high environment load (i.e. lead, hexavalent chromium, mercury, and cadmium) in a vehicle to be registered in EU Market on Jul. 1, 2003 or later. In the ELV directive, proactive use of trivalent chromium is recommended. Also in the electric and electronics industry, Waste Electrical and Electronic Equipment (WEEE) directive and Restriction on Hazardous Substances (RoHS) directive in Europe have reached final agreements to restrict the use of six specific hazardous substances including hexavalent chromium (Cr6+) for electrical or electronic devices as environmentally risky substances, even though the substances are separately collected from waste electrical or electronic devices. A printed wiring board is one of the subjects regulated.
In addition, due to increased awareness on environmental matters in recent years, it is afraid of that trivalent chromium used may be converted to hexavalent chromium by a wrong disposal processing or may be identified to be hexavalent chromium through a wrong procedure of analysis. According to such background, application of a copper foil for a printed wiring board free from chromium component itself has been investigated.
For example, Patent Document 3 discloses a metal foil having a bonding-promotion layer on at least one surface characterized in that the bonding-promotion layer contains at least one silane coupling agent and is chromium-free, and the base surface of the metal foil prepared under a bonding-promotion layer is not increased surface roughness. Or, as the metal foil characterized in that a base surface is free from a zinc layer or chromium layer attached, concept including chromium-free copper foil is disclosed. One of the metal foils disclosed is provided with a metal layer composed of a metal selected from the group consisting of indium, tin, nickel, cobalt, brass, bronze or a mixture of at least two of these metals between the base surface of the metal foil and the bonding-promotion layer of the metal foil. Another one metal foil disclosed is provided with a metal layer composed of a metal selected from the group consisting of tin, mixture of chromium and zinc, nickel, molybdenum, aluminum or a mixture of at least two of these metals between the base surface of the metal foil and the bonding-promotion layer of the metal foil.